1. Field of the Invention
The present invention relates to an electrophotographic photoconductor having an undercoat layer between a substrate and a photosensitive layer and a method of producing the same, and particularly, to the undercoat layer and a method of forming the same.
2. Description of the Related Art
The electrophotographic image forming process utilizing a photoconductor having photoconductivity, in general, is one of the image recording methods utilizing a photoconduction phenomenon of the photoconductor. More specifically, an image is formed by the steps of first uniformly charging the surface of the photoconductor by means of corona discharge in darkness, subsequently irradiating the charged surface of the photoconductor with an image light thereby selectively dissipating the charge of a light exposed portion of the photoconductor for forming an electrostatic latent image in an unexposed portion thereof, and developing the electrostatic latent image into a visible image by making toner particles, which are colored and charged, adhere to the electrostatic latent image by means of an electrostatic attractive force or the like.
In the sequence of the image forming process, the photoconductor is required of basic properties which include uniform chargeability to a predetermined potential in darkness, excellent charge-preservability for lower discharge, high photosensitivity such as to quickly start discharging in response to the light irradiation and the like. The photoconductor is further required of easy elimination of static charge on the surface thereof, and low residual potential and high mechanical strength of the surface thereof. In addition, the photoconductor must also present good flexibility, small variations in the electric properties including chargeability, photosensitivity and residual potential despite repeated use thereof, and good resistance to heat, light, temperature, moisture and ozone degradation.
The photoconductors currently used and giving considerations to the aforementioned properties are constructed such that the photosensitive layer is formed on the substrate having photoconductivity. Unfortunately, however, the aforesaid photoconductor is susceptible to carrier injection from the substrate into the photosensitive layer such that the charge on the surface of the photoconductor may be microscopically dissipated or decayed. This will result in the production of a defective image. There has been suggested a photoconductor wherein the undercoat layer is interposed between the substrate and the photosensitive layer in order to solve such a problem, cover a surface flaw of the substrate, improve the chargeability of the photoconductor and enhance adhering and coating properties of the photosensitive layer with respect to the substrate.
In the prior-art undercoat layer composed of a resin material alone, examples of a usable resin material include polyethylene, polypropylene, polystyrene, acrylic resin, vinyl chloride resin, vinyl acetate resin, polyurethane, epoxy resin, polyester, melamine resin, silicone resin, polyvinyl butyral, polyamide, and copolymers containing two or more of repeated units of these resins. The usable resin materials further include casein, gelatin, polyvinyl alcohol, ethyl cellulose and the like. Japanese Unexamined Patent Publication JP-A 48-47344(1973) discloses polyamide as a preferred resin material whereas Japanese Unexamined Patent Publication JP-A 52-25638(1977) discloses polyamide soluble in a solvent of halogenated hydrocarbon or alcohol as the preferred resin material.
The aforementioned photoconductor including the undercoat layer composed of the resin material alone suffers a relatively high residual potential and hence, a reduced photosensitivity. Therefore, the toner particles tend to adhere to a non-image area which does not bear the electrostatic latent image, thus resulting in the production of a defective image called a fogged image. Such a phenomenon is particularly frequently observed under conditions of low temperatures and low humidities. For elimination of such a phenomenon, the utilization of an undercoat layer composed of conductive particles or a resin material containing the conductive particles has been disclosed in, for example, Japanese Unexamined Patent Publications JP-A 55-25030(1980), JP-A 56-52757(1981), JP-A 59-93453(1984), JP-A 63-234261(1988), JP-A 63-298251(1988), JP-A 2-181158(1990), JP-A 4-172362(1992), and JP-A 4-229872(1992).
The aforesaid Japanese Unexamined Patent Publication JP-A 55-25030(1980) has disclosed an undercoat layer composed of conductive particles embodied by a metal such as Ag, Cu, Ni, Au, Bi or carbon, as well as an undercoat layer composed of a binder having the conductive particles dispersed therein. The Japanese Unexamined Patent Publication JP-A 56-52757(1981) has disclosed an undercoat layer containing titanium oxide.
The Japanese Unexamined Patent Publication JP-A 59-93453(1984) has disclosed an undercoat layer containing particulate titanium oxide surface-treated with tin oxide or alumina. The Japanese Unexamined Patent Publication JP-A 2-181158(1990) has disclosed an undercoat layer composed of a polyamide resin wherein particles of titanium oxide coated with alumina are dispersed. The Japanese Unexamined Patent Publication JP-A 4-172362(1992) has disclosed an undercoat layer containing a binder and particles of metal oxide, such as titanium oxide and tin oxide, which particles are surface-treated with a titanate coupling agent. The Japanese Unexamined Patent Publication JP-A 4-229872(1992) has disclosed an undercoat layer containing a binder and particles of metal oxide surface-treated with a silane compound or a fluorine-containing silane compound.
In the Japanese Unexamined Patent Publications JP-A 63-234261(1988) and JP-A 63-298251(1988), there are disclosed optimum mixing ratios between a white pigment and a binder in an undercoat layer principally composed of the white pigment, such as titanium oxide, and the binder.
The aforementioned undercoat layers and photosensitive layers are formed by a dip coating method featuring a relatively easy coating process, high productivity and low production cost. Since the forming of the undercoat layer is followed by the forming of the photosensitive layer, a resin material for the undercoat layer is preferably insoluble in a solvent for a coating fluid for photosensitive layer. In the light of the foregoing, a coating fluid for undercoat layer generally employs a resin material soluble in alcohol or water. The coating fluid is prepared by dissolving or dispersing the resin material therein.
In the case of the undercoat layer containing metal particles as the conductive particles, there is a problem that the photoconductor has a lowered chargeability which leads to a reduced image density when the photoconductor is repeatedly used.
In the case of the undercoat layer containing particles of metal oxide such as titanium oxide, an undercoat layer, which contains titanium oxide in a smaller amount and a binder in a correspondingly larger amount, has a great volume resistance, thus suppressing the transfer of carriers produced during the light irradiation. This leads to an increased residual potential of the photoconductor and hence, a defective image such as a fogged image results. Additionally, the photoconductor cannot offer satisfactory imaging characteristics because of serious decrease in the durability under conditions of low temperatures and low humidity.
Increasing the amount of titanium oxide may contribute to a smaller increase of the residual potential and to a smaller decrease of the durability under the low-temperature, low-humidity conditions. However, as repeatedly used over an extended period of time, the photoconductor tends to suffer an increased residual potential, particularly under the low-temperature, low-humidity conditions. As a result, the photoconductor cannot continue to maintain stable properties thereof over an extended period of time. On the other hand, the undercoat layer containing the binder in very little amount is decreased in the film strength and the adhesion to the substrate. This leads to a separation of the photosensitive layer and hence, the defective image results. In addition, because of serious decrease in the volume resistance, the photoconductor is lowered in the chargeability. Furthermore, titanium oxide presents a smaller affinity for the binder so that the dispersibility and can-stability of the coating fluid for undercoat layer is decreased. This results in inconsistent coating thicknesses and hence, excellent imaging characteristics of the photoconductor are not obtained.